The present invention relates to the fields of communications and optics. More specifically, the present invention relates to a method, apparatus, and system for reducing intensity modulation of acousto-optic tunable filters (AOTFs).
When an acousto-optic tunable filter (AOTF) is operating, radio frequency (rf) energy is used to bring about the coupling between two optical modes by the form of acoustic wave. During the acousto-optic coupling, unwanted rf intensity modulations are added to an optical signal that passes through the AOTF because of the frequency shifted optical wave components that come from static coupling, acoustic reflection, and cross talk between different rf frequencies. Intensity modulation can cause signal degradation by increasing bit error in a communication system (e.g., optical transmission system).
U.S. Pat. No. 6,253,002 and U.S. patent application Ser. No. 09/738,282, disclose a method for reducing intensity modulation using a single delay line. An example of a system using a single delay line to reduce the intensity modulation generated by a device (e.g., an AOTF) is shown in FIG. 1A. As illustrated in FIG. 1A, a system 100 (also referred to as a reflecting system herein) may include a device (e.g., AOTF) 110 which generates a corresponding intensity modulation, a delay line 120, and a mirror 130. The delay line 120, which is located between the device 110 and the mirror 130, is used in the system 100 to reduce the intensity modulation generated by the device 110. The length of the delay line 120 can be configured or adjusted based on the frequency of intensity modulation of the device 110 so that the phase difference between the intensity modulation generated directly from the device 110 and the intensity modulation that travels the delay line 120 can be made out of phase and hence the intensity modulation generated by the device 110 can be substantially reduced or canceled out, as illustrated in FIG. 1B. While the method disclosed in the related applications mentioned above can be effective for reducing intensity modulation generated by a single device, such a method may not be effective for reducing intensity modulation generated by multiple devices (e.g., multiple AOTFs) in a system in which each device may have non-negligible finite size and each device may have a different frequency of intensity modulation.
An apparatus is provided in which multiple devices are connected in series forming a signal path through which a signal propagates. Each device may have one or more operating frequencies and each operating frequency may generate a corresponding intensity modulation with respect to the signal. For one embodiment, the apparatus includes multiple delay lines through which the signal travels. The length of each delay line is configured to reduce the intensity modulation generated by each device based upon one or more factors including the one or more operating frequencies associated with each device and multiple null orders corresponding to different lengths of the delay lines.